1. Field of the Invention
This invention relates to a protective circuit for monitoring an object which can move between two end positions, for example a displaceable safety door of an electric machine.
2. Description of the Background
From experience and according to the state of the art, protective circuits of this type are well known. Here, protective circuits are provided to monitor the displacement of an object from a first end position to a second end position by means of end switch activation. For example, this type of protective circuit monitors safety doors, safety grids, or the like, of electric machines such as presses, punching devices, milling machines, lathes and lifts. It has to be ensured that the aforementioned machines can only be operated after correct closing of the safety door or the like by means of appropriate switching devices, for example in order to avoid injury hazards for operators. Normally two-handed operation is required for switching the machine into the respective dangerous mode, which is supposed to ensure that both the operator's hands are busy and that it is impossible for example to reach into the dangerous area with one or the other hand. However, at the same time, it is to be made sure that the operator cannot enter the dangerous area with other parts of the body, like for instance his legs, or with his clothes. However, it is also possible that this type of machine is activated through one hand activation of a switch, particularly a push button, so that the operator's other hand might be able to reach into the dangerous area. For this reason, machines of this type have safety doors, safety grids, light curtains or the like, which only allow the machine to be operated when these components are either placed in the correct closed position (safety door or grid) or correctly switched on (light curtain). With such protective circuits, it is necessary that any manipulation of the object to be moved or the associated end switches and machine activating device be made impossible. Furthermore, it is desirable that the protective circuit works faultlessly so that only under these circumstances a connection with the consumer, i.e. switching on of the dangerous operating mode, is allowed, but that on the other hand, in case of kind of failure occurs inside the circuit, the connection is reliably avoided. Electric protective circuits known thus far either do not meet all safety criteria or they are so complicated and their construction so costly that they require very high expenditures and that the complexity of construction has in fact to be considered a safety hazard.
EP-A-105 054 presents a protective circuit for monitoring an object which can move between two end positions, for example a displaceable safety door of an electric machine with at least two end switches that monitor the end position of the object.